It's not possible to share them across threads, so in case we decide to use multithreading in the future, it's best to mark them as thread_local right away, even if it's a no-op right now.
- Store Emscripten cache inside node_modules/.em_cache. Docker image ships without LTO libs, so Emscripten has to rebuild stdlibs on every build otherwise.
- Merge webp_enc + webp_dec build scripts. Core libwebp library is same in both cases, so there's no point in storing and building two copies of it.
